Conventional hot dip galvannealed steel sheet, which can be used for manufacturing automobiles or buildings, can exhibit excellent coating adhesion and corrosion resistance characteristics after coating. In automobile applications, for example, there may be a demand for such steel sheet having deep drawability. Thus, large amounts of hot dip galvannealed steel sheet formed using ultra-low carbon steel sheet as a sheet material may be used. In such steel sheet formed using ultra-low carbon steel, a corrosion resistance of a bare sheet and of scratched parts of coatings may not be sufficient. Further, it may be difficult to achieve both suppression of powdering and suppression of flaking when working such steel sheet, and flaws in appearance at the time of electrodeposition coating may occur easily.
A hot dip galvannealed steel sheet exhibiting excellent corrosion resistance which includes steel sheet having a first layer made of a Zn—Fe alloy layer and a second layer made of 8 to 15% Fe, 0.1 to 2% Ni, and 1% or less Al is described, e.g., in Japanese Patent Publication (A) No. 9-3417. Further, a method for producing hot dip galvannealed steel sheet exhibiting excellent corrosion resistance, characterized by preplating the surface of a steel sheet with 0.2 to 2 g/m2 of Ni, then rapidly heating the sheet to 430 to 500° C., hot dip coating the sheet in a galvanization bath containing Al in an amount of 0.05 to 0.25%, wiping the sheet, then heat treating the sheet at 470 to 550° C. for 10 to 40 seconds to promote alloying is described, e.g., in Japanese Patent No. 2783452. The above-referenced Japanese Patent Publication (A) No. 9-3417 and Japanese Patent No. 2783452 describe hot rolled low carbon Al killed steel sheet, and do not describe use of ultra-low carbon steel sheet which can exhibit improved deep drawability.
Ultra-low carbon steel sheet can exhibit a higher degree of cleanliness of ferrite grain boundaries, uneven progress of alloying, and easy growth of the Γ layer as compared with low carbon steel sheet. Thus, certain processes applicable to low carbon steel sheet may not be applicable to ultra-low carbon steel sheet. For example, Japanese Patent Publication (A) No. 9-3417 and Japanese Patent No. 2783452 cited herein above do not describe advantages relating to workability and coating behavior.
For example, a hot dip galvannealed steel sheet obtained by hot dip coating and alloying a sheet in a bath containing less than 0.2% of Al and 0.01 to 0.5% of Ni to give a coating containing 8 to 13% Fe, less than 0.5% Al, 0.02 to 1% Ni, and the balance Zn, and having a Γ layer thickness of the base iron boundary of 0.5μ or less, is described, e.g., in Japanese Patent No. 2804167. However, Japanese Patent No. 2804167 describes only a low carbon steel sheet and does not describe any use or advantages of ultra-low carbon steel sheet. Applying certain processes such as those described herein below to such low carbon steel sheet may produce a Γ layer thickness which may not be 0.5μ or less, and the resulting corrosion resistance, workability, and coatability may also be insufficient.
A method for producing hot dip galvannealed steel sheet which includes plating ultra-low carbon steel sheet with 20 to 70 mg/m2 of Ni, then annealing, hot dip galvanizing, and galvannealing it is described, e.g., in Japanese Patent No. 2800285. However, steel sheet produced using this method may not improve corrosion resistance and, further, may not lead to sufficient workability.
A hot dip galvannealed steel sheet exhibiting excellent slidability and coatability, obtained by plating steel sheet in a hot dip galvanization bath containing 0.1 to 0.2% Al and 0.04 to 0.2% Ni, alloying it by heating at a rate of 10 to 20° C./s temperature rise, and covering 1 to 40% of the surface with a 1 to 10 μm ζ layer, is described, e.g., in Japanese Patent No. 3557810. However, steel sheet formed using this procedure may likely not exhibit sufficient workability including, e.g., insufficient anti-powdering properties and corrosion resistance.
Plating of a steel sheet in a hot dip galvanization bath containing Al to which Ni and at least one of Pb, Sb, Bi, and Sn is added, and alloying under predetermined conditions to obtain hot dip galvannealed steel sheet containing 0.1 to 0.25% Al, 6 to 18% Fe, 0.05 to 0.3% Ni, and 0.001 to 0.01% of at least one of Pb, Sb, Bi, and Sn, is described, e.g., in Japanese Patent No. 3498466. However, this process uses a bath containing four elements and control of such bath may be difficult. Further, dross which can include Ni and Al may be easily formed in the bath. When such dross is caught up in the plating layer, it can lead to deterioration of corrosion resistance, and thus may not be desirable.
For example, ultra-low carbon steel sheet containing Ti can exhibit excellent deep drawability and ductility properties over a wide range of compositions. However, when hot dip galvanizing and further alloying such steel sheet, the Ti in the steel can lead to cleaning of crystal grain boundaries, such that an alloying reaction may be promoted at the crystal grain boundaries. As a result, an outburst reaction may occur easily, leading to overalloying deterioration of anti-powdering properties.
To address such problems, a method for producing hot dip galvannealed steel sheet which includes adding Nb together with Ti so as to control the alloying reaction occurring at the crystal grain boundaries, and thereby improving anti-powdering properties, has been described, e.g., in Japanese Patent Publication (B2) No. 61-32375, Japanese Patent Publication (A) No. 59-67319, Japanese Patent Publication (A) No. 59-74231, and Japanese Patent Publication (A) No. 5-106003. Such methods include adding Nb to Ti, but the addition of Nb can be costly, so it may not be economical.
To improve the anti-powdering properties of Ti-containing ultra-low carbon steel sheet without adding Nb, controlling a steam atmosphere during a cooling process after recrystallization annealing, which can cause crystal grain boundaries to oxidize and suppress outburst at the time of the alloying reaction, is described, e.g., in Japanese Patent Publication (A) No. 10-287964. Oxidation may be difficult to control in such a procedure, and the plating appearance may likely be adversely affected.
A method for producing steel sheet which includes raising the concentration of Al in the hot dip plating bath to between 0.12% and 0.2% or higher and creating locally elevated Al concentration phases at the base iron-plating boundary is described, e.g., in Japanese Patent Publication (A) No. 8-269665. However, the plating layer produced by such method may easily become uneven and the appearance may easily deteriorate.
For example, when hot dip galvannealed steel sheet is used for automobile body panel applications, an uneven appearance of galvannealing often remains even after painting the automobile. Thus, an extremely high quality of appearance may be desirable. Most of this unevenness can result from unevenness of an oxide film of the plated sheet material, unevenness of the fine ingredients, and other unevenness arising from previous processes, although the specific causes may generally be difficult to identify. Solutions for preventing such unevenness may therefore be difficult to achieve. For example, the publications described herein above do not provide guidelines for obtaining an excellent appearance of steel sheet which may be suitable for use in automobile body panels.
When producing hot dip galvannealed steel sheet, an Fe—Al—Zn alloy layer (e.g., a barrier layer) may generally be formed in a hot dip galvanization bath at the base iron-plating boundary. Such alloy layer may be removed by later heat treatment, and an Zn—Fe alloy layer in which Al is diffused can be formed. The Fe—Al—Zn alloy layer can play an important role in controlling the subsequent Zn—Fe alloying reaction and securing plating adhesion. However, the speed of formation of the Fe—Al—Zn alloy layer can be affected by surface conditions of the plated sheet material, flow of solution in the plating bath, etc. Fine differences in thickness of the Fe—Al—Zn alloy layer can have a direct effect on the alloying reaction behavior, and fine unevenness in plating appearance may be induced. Thus, it may not be easy to produce hot dip galvannealed steel sheet which exhibits an excellent appearance.